You don't mention what machine you are referring to, but most lawn tractors are DESIGNED to cut out if you reverse with the blades engaged - it's dangerous and usually a tractor has a particular procedure to be followed if there is no alternative - for instance on Craftsman tractors there is a position on the ignition switch that allows it.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lawn mowers.